FIG. 1 illustrates a circuit diagram of a conventional battery charging system 100. The battery charging system 100 includes a normal current charging path 102 for transferring a normal charging current Inormal from a power supply terminal 106 to charge a battery pack 108 when the battery pack 108 is in the normal state. The battery charging system 100 also includes a small current charging path 112 for transferring a relatively small current Ismall when the battery pack 108 is in the zero-volt (ultra low volt, under voltage protection) state.
When the charging system 100 starts to operate, the small current charging path 112 is enabled by a control signal 114, and the normal current charging path 102 is disabled by a control signal 104. As such, the small current Ismall flows through the charging path 112 to charge the battery pack 108. Additionally, the charging system 100 receives a feedback signal 110 indicative of a battery voltage Vbat of the battery pack 108. If the battery voltage Vbat is less than a voltage threshold Vth of the normal charging process (including the pre-charging process and the quick-charging process), the battery pack 108 is in the zero-volt state and is charged by the small current Ismall. If the battery voltage Vbat is greater than the threshold voltage Vth, the small current charging path 112 is disabled, and the normal current charging path 102 is enabled. Thus, the battery pack 108 is charged by the normal charging current Inormal, and the charging system 100 enters the normal charging process.
Usually, the small current charging path 112 includes a power resistor 118 coupled in series with a charging switch 116, to limit the charging current to a relative small current Ismall. The power dissipation on the small current charging path 112 may be relatively high, which increases the system power consumption. In addition, the size of the printed circuit board (PCB) for the battery charging system 100 may be relatively large, increasing the cost of the charging system 100.